1. Field of the Art
The present invention relates to a process for the production of a polyarylene sulfide (hereinafter referred to as "PAS"). More particularly, the present invention relates to a novel process for the production of a high-molecular-weight linear PAS having a melt viscosity of at least 1,000 poises (P) at low cost without using a crosslinking agent or an organic acid salt.
In recent years, thermoplastic resins having increasingly high heat resistance have been sought as materials for the production of electric equipment parts and automobile parts.
PAS has properties satisfying this requirement, but a product having a sufficiently high molecular weight cannot be obtained. Moreover, PAS represented by polyphenylene sulfide, has been problematic in that it is very difficult to use to obtain fibers and films for which high strength is required and molded articles for which high impact strength is required.
The present invention provides a process in which this difficulty is overcome and PAS having a remarkably high molecular weight is prepared at low cost.
2. Description of the Prior Art
As a typical example of the process for the production of PAS, a process in which a dihalo aromatic compound such as dichlorobenzene is reacted with sodium sulfide in an organic amide solvent such as N-methylpyrrolidone, as disclosed in Japanese Patent Publication No. 3368/70 can be mentioned. PAS prepared according to this process has a low molecular weight and a low melt viscosity, and it is therefore difficult to form a film, a sheet, a fiber or the like from this PAS.
Accordingly, various improvements of this process for obtaining PAS having a high polymerization degree have been proposed. As a most typical example, there can be mentioned a process in which an alkali metal carboxylate is used as a polymerization catalyst in the above-mentioned reaction, as disclosed in Japanese Patent Publication No. 12240/77. According to this process, it is taught that the polymerization catalyst should be used at least in an amount equimolar to the amount of the alkali metal sulfide. Furthermore, according to this process, in order to obtain PAS having a high polymerization degree, it is necessary that an expensive polymerization catalyst such as lithium acetate or sodium benzoate be used in a large amount, whereby the manufacturing cost of PAS is increased, and the process is disadvantageous from the industrial viewpoint. Moreover, this process involves the risk of environmental pollution because large quantities of organic acids are contained in waste water left after recovery of PAS from the polymerization reaction mixture, and large expenses are required for eliminating this risk. Therefore, it is considered that this process is disadvantageous from the economical viewpoint.
As another process for preparing PAS having a high polymerization degree, there has been proposed a process in which a polyhalo aromatic compound having a functionality of at least 3, such as trichlorobenzene, is added as a crosslinking agent or branching agent during the polymerization or at the terminal stage of the polymerization (see, for example, Japanese Patent Application Laid-Open Specification No. 136100/78). According to this process, PAS having apparently a high molecular weight, for example, a melt viscosity of 20,000 P or more can be prepared, but this PAS has poor the spinnability or stretchability property because this PAS is a highly crosslinked or branched polymer. Accordingly, it is difficult to form this PAS into a film or fiber, or even if a shaped article is obtained, since the molecular chain is short in principle, the shaped article is mechanically very brittle.
Under with this background, we carried out research on the polymerization mechanism in the simple polymerization system between an alkali metal sulfide and a dihalo aromatic compound with a view to developing a process for preparing linear PAS having a high melt viscosity at a low cost without using a polymerization catalyst such as an alkali metal carboxylate. As a result, we have found that if the polymerization conditions, especially the amount of water present in the system and the polymerization temperature, conducted in the initial stage of the polymerization are made considerably different from those used in the latter stage, linear PAS having a high molecular weight corresponding to a melt viscosity of 1,000 to 8,000 P can be produced without using an assistant. We proposed this process in Japanese Patent Application No. 126725/84.
Not only in the conventional polymerization processes but also in the process proposed in Japanese Patent Application No. 126725/84, in the case of an ordinary reaction vessel material, in order to obtain a PAS having a melt viscosity of at least 1,000, it is necessary to control the amount of water present in the system in the initial stage of the polymerization within a relatively narrow range of 0.5 to 2.4 moles per mole of the alkali metal sulfide. Furthermore, since the alkali metal sulfide industrially available is in the form of a trihydrate, a pentahydrate or a nonahydrate, that is, a salt containing a large amount of water, it is necessary to strictly control the effective water content by removing a large amount of excessive water before initiation of the polymerization.